Question

How to check if user has logged in to SharePoint Online or not from a web application?

An approach is search for cookies of SharePoint in local machine, but it throw a security problem.

Any solution?

Answer 1

I have found the solution, exactly a trick that come from jQuery.getScript()

Using a function that return a string, and place one in web page, one in a .js file and upload to SPO doccument library. For example:

This function put in local web page:

function whereAmI(){
    return "local";
}

This function save in a xxx.js file and upload to SPO:

function whereAmI(){
    return "remote";

Notice those two function have same name.

Then in local web page, insert the following code:

jQuery.getScript("https://xxx.sharepoint.com/sites/xxx/DocLib/xxx.js", function () {
            if (whereAmI() == "local") // if not logged in, getScript failed to get xxx.js, so the page call local whereAmI
                // do something;
        });

That’s it.